Local neuronal density in the brain varies enormously across species as well as across sites within a brain. What are the implications for how much energy is available to each neuron? Compared to smaller neurons, do larger neurons require more energy (as is expected in the literature), receive more energy (in case that is constrained by the vasculature), or neither?

To answer those questions we analyzed the vascular fraction and the density of vasculature-associated cells in relation to densities of neurons, glial cells and synapses across all structures in the rat and mice brain. We confirm that published local glucose utilization rates are directly relatedto measured vascular fraction, which allows inferring metabolic rate in fixed brain tissue. This talk will present data that support the notion that neuronal metabolic rate is constrained by brain vasculature, and that the number of synapses per neuron in a given brain is constant, regardless of the size of the neuron.